Biopolyesters are polymer materials produced by microorganisms from biological resources such as sugars and fatty acids. The polyesters have a unique property called biodegradability such that they are decomposed and assimilated by the action of microorganisms in the environment and are expected to be utilized as one of biodegradable polyesters, like aliphatic polyesters obtained by chemical synthesis. Poly(3-hydroxybutyrate), a representative example of the biopolyester, is produced with an enzyme of a microorganism, so that it consists of 100% of a structural unit having a stereocenter of the (R)-form ((R)-3-hydroxybutyrate); and the poly(3-hydroxybutyrate) is a polymer material that cannot be produced by chemical synthesis.
Also, the polyester has a feature that it has a melting temperature of 180° C. and hence is thermoformable. However, the biopolyester undergoes a reduction in molecular weight due to thermal decomposition reaction in a temperature region of 160° C. or more. The reduction in molecular weight causes a reduction in strength of material, so that this property is a great drawback to utilization of biopolymers. When considering melt-forming of biopolyesters, it is necessary to develop a methodology by which the reduction in molecular weight of the biopolyester can be minimized within the temperature region of up to 180° C., which is its melting point.
It has been reported that a main reaction of the thermal decomposition of a biopolyester is a molecular weight reduction due to random cleavage of its molecular chain (Non-Patent Document 1).
As a prior art relating to prevention of thermal decomposition reaction of a biopolyester that has a melting temperature and a thermal decomposition temperature close to each other, there has been used a method of adding a compound that shows a plasticizing effect to a sample and lowering the melting temperature of the sample. One example thereof is a method in which a low molecular weight compound is physically added to the sample (Non-Patent Document 2). However, a molding to which a low molecular weight compound is added has problems that the addition of the low molecular weight compound results in exhibiting a property (such as a reduction in melting temperature, a reduction in strength of material, or the like) that is different from an original property of the biopolyester itself and that elution of the additive during duration of use causes deterioration of performance.
Another method is synthesis of a copolyester that contains a different molecular structure that is chemically added to its molecular chain (Non-Patent Document 3). In this method, deterioration of performance with time during duration of use is not caused but a physical property different from that of a homopolymer is exhibited due to the copolymerization composition. Therefore, there is a problem that the performance expected for the homopolymer is lost.
In various synthetic polymers, residues of a polymerization catalyst (in particular, residual metals) are known to promote a thermal decomposition reaction of polymer materials in the process of hot forming (a reverse reaction of polymerization reaction). Accordingly, there have been established synthesis technologies and purification technologies that decrease a polymerization catalyst that is contained or remaining in the synthesized polymer material as much as possible.
On the other hand, biopolyesters produced by microorganisms are synthesized in the form of particles in cells of the microorganisms by polyester synthetic enzymes in the cells of the microorganisms. The produced biopolyesters have to be purified by solvent washing or solvent extraction to separate them from proteins such as polyester synthetic enzymes and other biomass.
Methods of separating biopolyesters thus far proposed include a method that comprises extracting a biopolyester from cells of a microorganism with a solvent into which the biopolyester is soluble and separating the solution from cell residues, and a method that comprises removing cellular substances other than the objective polymer by an enzyme treatment or the like. In the purification methods using solvents, chloroform or methylene chloride (Patent Document 1), pyridine (Patent Document 2), dioxane (Patent Document 3), and so on are used as extraction solvents. On the other hand, regarding the method that comprises removing the cellular substances other than the objective polymer using an enzyme or the like, a method of separating and purifying the objective polymer by treating microbial cells with an alkaline solution of sodium hypochlorite has been proposed as disclosed in Non-Patent Document 4. Also, in Non-Patent Document 5, there is disclosed a method that comprises adding lysozyme to a microbial cell suspension, sonicating the suspension, loading the sonicate on glycerol, and purifying the objective polymer by centrifugation due to a difference in specific gravity. In Patent Document 4, there are proposed various methods that are combinations of molecular weight reduction of nucleic acid-related substances by heat treatment, digestion with a protease such as alkalase, digestion using a surfactant such as sodium dodecyl sulfate, and so on. Further, a method that comprises a treatment with surfactant and diluted alkaline solution of sodium hypochlorite has been proposed.
However, with these methods, it is very difficult to purify biopolyesters with high purity. Further, various elements constitute various kinds of compounds in cells of microorganisms, so that it is very difficult to remove all the impurities from the biopolyesters. For the above-mentioned reasons, there have been made no studies to identify residues (in particular, residual metal ions) that promote thermal decomposition reaction of biopolyesters upon hot forming and to elucidate the effects thereof.    [Patent Document 1] JP 57-65193 A    [Patent Document 2] U.S. Pat. No. 3,044,942 A    [Patent Document 3] JP 63-198991A    [Patent Document 4] JP 60-145097 A    [Non Patent Document 1] Polym. Degrad. Stabil., 6 (1984)127-134    [Non Patent Document 2] Japanese Journal of polymer science and technology 47 (1991) 221-226    [Non Patent Document 3] Phys. Technol., 16 (1985) 32-36    [Non Patent Document 4] J. Gen. Microbiology 19 (1958) 198-209    [Non Patent Document 5] J. Bacteriology 88 (1964) 60-71